It is known to provide a plurality of light emitting diodes (LED's) to visually indicate, by "on" or "off" conditions of the LED's, the status of various operating components of a system, such as machine functions. For example, each LED can represent a cam position, a relay contact closure, a temperature limit, and any other physical or electrical parameter used in machine control systems, typically, each LED is connected in a circuit interfaced with a control circuit which either electrically operates or monitors the particular machine function. Monitoring the operational state of the machine function is accomplished by detecting the "on" or "off" status of the control circuit associated with it.
One commercially-successful circuit status indicating device useful for monitoring equipment during operation, and also adapted for other applications as well, is disclosed in U.S. Pat. Nos. 4,211,956 and 4,342,947. In order to interface with machine control circuits operated at conventional AC voltage levels, for instance, of 120 volts AC, this device employs a phase controlled silicon controlled rectifier (SCR) circuit connected to LED's so as to substantially reduce the electrical power and heat generation in the system. Specifically, the SCR circuit limits conduction of rectified AC energy through the LED's and through corresponding current limiting resistors connected in series with the LED's for limiting the current through the LED's. The SCR circuit thereby operates low DC voltage LED's from high AC voltage sources, i.e. the monitored machine control circuits, with a minimum of power and heat generation in the current limiting resistors.
The patented circuit status indicating device works satisfactorily when interfaced with machine control circuits through so-called "hard" contact switches, such as relay contacts, that are non-conducting when in an "off" status. However, the current trend in machine control technology is toward use of programmable controllers and proximity detectors which utilize solid-state switches, such as triacs, in their outputs. These so-called "soft" contact switches commonly employ a series resistor/capacitor network, a so-called snubber circuit, in parallel with their triacs for transient protection. Such snubber circuit continues to conduct some fraction of the current of the machine control circuit even when it is in an "off" status. As a consequence, the circuit status indicating device of the aforesaid patents is unable to switch its respective output LED completely off, preventing an operator from using the indicating device to ascertain the correct operational status of the machine function being monitored.